He is too relaxed in the D-zone and plays much softer than he should. Add in the contract, and Calgary's expected return and it becomes a tough sell. Also the fact that he didn't last in Boston isn't alarming in itself, but the fact that they were willing to give him up for nothing but draft picks is. You just don't see teams giving up high end NHL players for magic beans very often (Trouba/Fowler/Lindholm come to mind)
The other thing is he has had a "sheltered" roll with Brodie/Gio taking the tough matchups; Hamilton hasn't exactly looked consistent against the middle 6 matchups.
Well Brodie took heat, but being that he is a better player the assumption was that he will rebound. With Hamilton, he has been productive yet his play isn't reassuring and often doesn't add up to what you see on the stat line. Being that Hamilton hasn't demonstrated that same high level of play, there is concern about how good he really is.
To relate this to the Red Wings think of a Howard vs Mrazek comparison from last season. If Mrazek had a bad game or two, the reactions were, "bad games happen to everyone including the best player." When Howard had a bad game the reactions were, "See Howard sucks!" The thought process being that when a perceived lesser player struggles it confirms what we already believe vs a higher end player going through a slump, its dismissed as just that, a slump.
I guess this is all just a long way of saying, that Brodie has demonstrated a high level of play so the assumption is he will rebound. Hamilton, having never demonstrated that same level (even against lesser competition), does not get the benefit of the doubt.